Implantation marks the initial physical connection between mother and the conceptus during pregnancy, and its success is greatly dependent upon the function of the embryonic cells, the trophoblast, that establish themselves in the uterine wall and give rise to the placenta. Little is known about the biochemical basis for the invasive activity of the trophoblasts during early implantation because of the complexity of the uterine endometrium, its inaccessibility to experimental study and the paucity of material comprising the embryo itself. This laboratory is endeavoring to develop methods that will directly address biochemical problems of mammalian development, given the limited quantities of available biological materials. The objective of the proposed studies is to examine the developmentally regulated interaction of trophoblast cells with components of the uterine extracellular matrix (ECM), focusing on fibronectin which is a major component of the endometrial ECM that is infiltrated by differentiated trophoblasts early during interstitial implantation. The fibronectin-binding characteristics of trophoblast cells at the surface of developing blastocysts will be studied and the corresponding cell surface receptors will be identified. All of these studies will focus on our finding that blastocysts will outgrow in vitro on fibronectin coated surfaces. This molecule has been widely studied in tissue culture cells, generating much information about its structure-function relationships and its binding to a specific cell surface receptor glycoprotein, integrin. We will therefore be able to generate biochemical and immunological probes to test the hypothesis 1) that trophoblast binding to fibronectin is mediated by a fibronectin-specific integrin, and 2) that integrin is developmentally regulated during the onset of attachment competence of the mouse blastocyst. Experiments will also be conducted, using similar methods, to determine whether specific receptors for fibronectin or various other adhesion-promoting ECM components (laminin, vitronectin, hyaluronic acid) are ligand-induced or constitutively expressed by trophoblast cells. Finally, the findings of the in vitro studies will be tested in a newly developed culture system wherein embryos are co-cultured with an acellular preparation of ECM isolated from the receptive endometrium to determine the relative role of trophoblast-fibronectin interactions within the complex ECM encountered in utero.